zlib: use Z_DEFAULT_STRATEGY by default instead of Z_FIXED

Since Envoy::Compressor::ZlibCompressorImpl::CompressionStrategy is
simply static_cast'ed to uint64_t the Standard strategy (4)
becomes Z_FIXED (4 as well). This basically disables the use of
dynamic Huffman codes when the gzip filter is configured to use
default values.

Make the Standard strategy equal to 0 to translate to
Z_DEFAULT_STRATEGY.

Contributes to envoyproxy#8448

Signed-off-by: Dmitry Rozhkov <dmitry.rozhkov@linux.intel.com>

source/common/compressor/zlib_compressor_impl.h

@@ -50,7 +50,7 @@ class ZlibCompressorImpl : public Zlib::Base, public Compressor {
     Filtered = 1,
     Huffman = 2,
     Rle = 3,
-    Standard = 4,
+    Standard = 0,
   };
 
   /**